A novel integrated self-powered brake system for more electric aircraft

Yaoxing SHANG,Xiaochao LIU,Zongxia JIAO,Shuai WU

School of Automation Science and Electrical Engineering,Beihang University,Beijing 100083,China

1.Introduction

Aircraft brake systems play a major role during aircraft takeoff and landing.Their major functions include anti-skid brake,brake during the take-off,and brake after the full retraction of landing gears.1,2Hydraulic brake systems are widely applied in an aircraft due to their high power density and robustness,and the fact that they use the same hydraulic power supply as for the primary flight control system.An engine driven pump(EDP)supplies high-pressure oil to brake actuators through a complex system of pipelines.3A pressure control servovalve is typically used in these circumstances,and is located near a landing gear.4The length of pipelines between the EDP and brake actuators is generally between 8 and 10 m,and can go up to 20 m in the case of a large aircraft.In order to improve the system reliability,double redundancy is typically utilized.These complex pipelines and their annexes are very heavy and may even cause serious vibration and oil leakage problems.They are potentially the major risks to the aircraft safety.5Due to the large number of separate components,it is difficult to improve a hydraulic brake system.

More electric aircraft(MEA)technology,6,7especially the power-by-wire technology,have become one of the main research areas over the years.Many researchers have focused on the study and development of electro-hydraulic actuator(EHA)and electro-mechanical actuator(EMA)brake systems.8,9An EHA system10was introduced by Habibi and Goldenberg,which uses an electric motor driving a hydraulic pump to generate high-pressure oil for brake actuators.In this system,the electric motor of the EHA always works under the peak power condition during the entire braking process.For large passenger aircraft and transport aircraft,the electric motor is heavily burdened,which always causes serious cooling problems of the motor.An EMA11using one ball screw to push brake disks was introduced by Garcia et al.EMA systems are used in commercial aircraft including B787.However,the disadvantages of an EMA including the power density of the electric motor and the mechanical jamming are still evident and difficult to solve.

Recently,many studies about new brake systems have also been conducted.An example is the magnetorheological(MR)brake system12,13discussed by Park et al.They concluded that the MR brake system has a better performance than that of a conventional hydraulic brake system in many aspects.Another example is the electrorheological(ER)brake system14,15presented by Lee et al.They discussed the basic control method of the ER brake system and indicated that the new brake system has a good research value and application prospect in areas of automobile brakes and aircraft brakes.However,both MR and ER brake systems still stay at the stage of laboratory veri fication due to the limitations of a small brake torque and a complex structure.

This paper presents an integrated self-powered brake system(ISBS)as a new solution for the development of aircraft brake systems.In the ISBS,one hydraulic pump geared to the main wheel is used to convert a small portion of the kinetic energy of landing aircraft into hydraulic energy,and supplies the energy to the brake actuators.The ISBS is highly integrated and placed close to the main wheel.The complex system of pipelines between the aircraft EDP and the brake actuators is replaced with electric wires.

Although the concept of kinetic energy recovery is widely considered,so far most of the studies have focused on vehicles,which are referred to as regenerative brakes16–18for electric vehicles(EVs)or hybrid-electric vehicles(HEVs).These methods capture the kinetic energy in the braking mode by having an electric motor coupled with a wheel to charge batteries or capacitors.The captured energy is subsequently used to accelerate vehicles.Given this,the vehicle emission can be reduced while the driving range is improved,especially for periodic acceleration-deceleration cycles.The existing research19,20on regenerative braking has one thing in common,i.e.,the recovered energy is used to drive a vehicle so as to increase energy efficiency.To accomplish this goal,as much energy as possible needs to recover,so an energy accumulation unit with a high power density and a light weight is necessary.21The ISBS is different from a regenerative brake as it is used to provide the energy for braking instead of driving a vehicle.This goal is unrealistic for aircraft because the kinetic energy of a landing aircraft is too much to be recovered.Different from regenerative braking,the ISBS only recovers a small percentage of the kinetic energy which is just enough for the brake system.Therefore,the ISBS will achieve a very small volume as well as a light weight.As a result,the original central hydraulic power system and the heavy pipelines for the brake system can be significantly simplified or removed.

In this paper,the hydraulic schematic diagram of the ISBS is presented and an ISBS prototype is designed.A feedback linearization control algorithm is designed to fulfill the antis kid control.Software simulations are conducted to verify the performance of the ISBS.Then,a ground brake test bench is set up to carry out prototype experiments.Finally,the experimental results of the ISBS are presented.

2.Working principle of the ISBS

For the first time,this paper proposes the concept of a self powered brake system for aircraft and describes the working principle of the new system.As shown in Fig.1,when an aircraft lands,with the help of the energy conversion device,the rotational mechanical energy of the wheels is converted into hydraulic energy,which is supplied to the brake actuators or stored by the energy storage device.The brake actuators then generate a brake torque to finish the brake function.The stored energy will be supplied to the brake system when the wheels rotation speed reduces to a certain value.In this paper,the energy conversion device is a hydraulic pump and the storage device is an accumulator.

The self-powered brake system uses a proper hydraulic pump to recover a small part of the kinetic energy from a landing aircraft during its braking process.This hydraulic pump generates local hydraulic power for brake actuators.The specific hydraulic schematic diagram of the ISBS is shown in Fig.2,which consists of a constant-pressure hydraulic pump,a pair of transmission gears,a check valve,an accumulator,a servo-valve,and other basic hydraulic components.The hydraulic pump is linked with the main wheel through transmission gears.When the aircraft is taxiing on a runway,the main wheel will drive the pump to generate hydraulic power,which is supplied to the servo-valve to control the brake pressure.When the wheel angular velocity gradually decreases,the pump output flow will go down as well.Thus,a hydraulic accumulator is used to compensate the brake flow and maintain the brake pressure while the wheel angular velocity reduces to a low value.Besides,a shut-off valve is used to cut off the supply of high-pressure oil for the servo-valve when the aircraft is not in the braking process.In addition,the shutoff valve can avoid oil leakage caused by the pilot-operated stage of the servo-valve under its cutoff status.The ISBS is designed to be compact and can be situated in the landing gear next to the main wheel.It can supply hydraulic power to one or several wheels on the same landing gear bogie.Actually,a pressurized tank is designed to improve the suction and discharge characteristics of the hydraulic pump.In fact,in order to reduce the volume and weight of the ISBS,the accumulator and the pressurized tank are designed to be integrated.The tank is pressurized by introducing the high-pressure gas of the accumulator,which means the accumulator and the pressurized tank are pressurized with the same air cavity,so the volumes and weights of the accumulator and the pressurized tank are largely reduced.

When compared with the traditional hydraulic brake system,the ISBS obtains energy only from the aircraft wheel and works without airborne energy.Therefore,the original heavy and complex hydraulic pipelines can thus be removed.As a consequence,the aircraft economy and reliability are improved.Moreover,the ISBS has a better performance than that of the EMA system,because its actuators are the same as those of the traditional hydraulic system.The high-power electric motor is no longer needed,which is indispensable to both EMA and EHA brake systems.There is another unique advantage of the ISBS,i.e.,its self-powered characteristic.It makes an aircraft be able to brake as usual even in an emergency case of airborne power lost.The aircraft security is also better guaranteed.

The design objective of the ISBS is to optimize the aircraft brake system by improving its system modularization,economic efficiency,and practical reliability.The airborne application has strict requirements on the weight and volume.Therefore,different from vehicles or other kinds of energy recovery systems,the ISBS should adhere to the following design principles.

First of all,it only recovers a certain amount of energy,which just meets the demand of the brake system.When the aircraft brakes,the kinetic energy it dissipates is much more than the energy needed for the brake system.However,the more energy that is recovered and stored,the bigger and heavier the energy conversion and storage device will be.Therefore,the self-powered brake system is designed not to achieve a high energy recovery,but to realize the smallest and lightest energy recovery device when it meets the energy needed for the brake system.

Secondly,its work priority is energy conversion while energy storage provides some auxiliary function.In the ISBS,the energy conversion device is necessary,while the energy storage device mainly depends on the operational condition of the brake system and its leakage when the wheels rotation speed reduces under a certain value.Moreover,under the current technological condition,the energy density inside the energy conversion device should be higher than that inside the storage device.Given this,improving the instantaneous power output by the energy conversion device to meet the brake demand is helpful to reduce the weight of the self powered brake system.

Thirdly,minimizing system leakage can help to reduce the volume and weight of the self-powered brake system.Usually,existing aircraft hydraulic brake systems use a traditional pressure servo-valve as the pressure control component.The pressure servo-valve has a two-stage structure,in which the pilot-operated stage is a nozzle flapper and the main stage is a slide valve.Actually,the internal leakage of the pilotoperated stage accounts for most part of the brake system leakage and brings the system a huge loss of energy.This is the same for the self-powered brake system,whose miniaturization is mainly restricted to the problem of the system leakage.

As we know,aircraft brake systems play a major role during aircraft take-off and landing.Their major functions include the anti-skid brake,the brake on the take-off,and the brake after the full retraction of the landing gears.Actually,these normal brake functions can be achieved by the ISBS.According to our design,when the aircraft velocity is greater than 20 km/h,the recovered energy by the ISBS is enough for the braking system.On the contrary,when the aircraft velocity is less than 20 km/h,the recovered energy is not enough for the braking system,and the accumulator will supply additional oil.However,an aircraft is rarely taxiing at a velocity less than 20 km/h,and the aircraft no longer needs the anti-skid brake control function under the condition of low velocity,so that the brake flow is very small.Therefore,the ISBS basically covers all of the aircraft braking conditions.In addition,for insurance purposes,adding an auxiliary motor with very small power in the ISBS is a good choice,as shown in Fig.3.If the recovered energy is not enough for the braking system,the auxiliary motor will start to work to drive the pump,so that the ISBS can still work properly when the aircraft is in a very low velocity situation.Thus,the robustness of the ISBS in different working conditions is guaranteed.

Actually,the ISBS can be designed to be a redundant system,and there are two main backup modes.The first one is backed up by an airborne centralized hydraulic source,as shown in Fig.4.If the ISBS fails,the airborne centralized hydraulic source will be selected to supply the brake system with high-pressure oil.The second one is by adding a backup motor,as shown in Fig.5.The backup motor will drive the pump to generate high-pressure oil,if the mechanical structure of the ISBS fails.In the ISBS,the backup motor is completely different from those in the EHA and EMA.In the EHA and EMA,the motor needs to provide the peak power required by the brake system;however,the backup motor in the ISBS does not need to provide the peak power,which benefits from the energy management function of the accumulator.In general,the brake system needs the peak flow only when the actuator compresses the brake disks.In other normal processes,the flow needed by the brake system is just a fifth of the peak flow,so the accumulator can perform the function of energy management.During normal processes,the flow generated by the backup motor will not only satisfy the need of the brake system,but also charge for the accumulator.During the compressing process,the flow generated by the backup motor is not sufficient for the brake system,and then the accumulator acts as a hydraulic source for the brake system.Based on the energy management function of the accumulator,the peak power of the backup motor can be designed around one fifth of the peak power of the brake system,which will greatly reduce the volume and weight of the backup motor.In fact,the ISBS can make different levels of backup according to different working conditions,so as to ensure the normal security of the whole system in different failure modes.

3.System design of the ISBS

This section focuses on the calculation method for several key parameters of the critical components,including the displacement of the pump and the volume of the accumulator.Both the volume and weight of the airborne equipment are strictly controlled,which requires that the ISBS needs to be carefully designed.

3.1.Pump displacement

It is assumed that the motion of an aircraft is considered as a uniformly deceleration from landing to stopping which takes T seconds,as shown in Fig.6.The initial velocity of a landing aircraft is v0,and ω0is the corresponding initial wheel angular velocity.Thus,the wheel angular velocity ω during the braking process is depicted by Eq.(1),which means that it decreases from ω0to zero.

where t is the braking time.The pump is coupled to an aircraft wheel by means of transmission gears,so the output flow of the pump Qsis proportional to the wheel angular velocity ω during the braking process,as

where Dmis the displacement of the pump and η is the volumetric efficiency of the pump.i is the gear transmission ratio between the pump and the wheel.Based on the fact that the ISBS only recovers some energy just enough for the brake system,the oil volume generated by the pump is not less than the amount needed in the braking process,as

where Qfis the needed flow for the brake system,including the brake flow Qband the leakage flow of the servo-valve Qc,i.e.,Qf=Qb+Qc.In fact,the brake flow Qbis the required flow of the brake actuators.The rated brake flow Qbis considered as a constant.In fact,in the whole braking process,the instantaneous brake flow continuously changes around a certain value.In order to simplify the ISBS design,the average of the instantaneous brake flow is regarded as the rated brake flow Qb.

For Eq.(3),If Qs＞Qf,extra oil will either charge the accumulator or over flow from the safety valve.If the equation balances,there will be no over flow.If Qs＜Qf,the accumulator will compensate the deficiency of the brake flow.Eq.(3)is demonstrated in Fig.7,i.e.,the size of Region A is not less than that of Region B,in which Region A is the oil volume generated by the pump and Region B is the oil volume needed for the brake system.

3.2.Accumulator volume

The output flow of the pump will decrease gradually with the

slowdown of the aircraft wheel.If Qs＜Qf,the accumulator will compensate the de ficiency.Therefore,the effective volume of the accumulator Vais

where tarepresents the time when Qs=Qf.Eq.(4)is shown in Fig.8,i.e.,Vais not less than the size of Region C.

Table 1 Parameters of a large aircraft.

Table 2 Results of ISBS system design.

The related physical parameters of a large aircraft are listed in Table 1,based on which some design results of the ISBS are listed in Table 2.

4.Models of the aircraft and the ISBS

In this section,the system dynamic models of the aircraft and the ISBS are developed.The system model consists of the aircraft taxiing model and the ISBS model.

4.1.Aircraft taxiing model

The motion of the aircraft is simpli fied to a rectilinear motion model with a single wheel and a portion of the aircraft mass attached to it,as shown in Fig.9.This model is relatively simple,but retains the essential characteristics of the actual system.During the braking process,the engine for ceis neglected.The airframe is considered as a rigid body,and only the longitudinal motion is taken into account.

According to the second law of Newton,the aircraft taxiing model is described as

where v is the aircraft longitudinal velocity,Ffis the friction force between the tire and the runway,M is the aircraft mass supported by one wheel,J is the wheel moment of inertia,Tbis the brake torque,Tpis the torque applied to the wheel by the pump of the ISBS,Fdis the aerodynamic drag force,and Mris the rolling resistance moment.Actually,the pump torque Tpcan be neglected because it is much smaller than the brake torque,which is proven in the simulation.The aerodynamic drag force Fdand the rolling resistance moment Mrare described as

where Cdis the coefficient of the aerodynamic drag,and f0and f1are the curve fit parameters.The common ‘magic formula”,22as shown in Eq.(8),is used to describe the relationship between the friction force Ffand the slip ratio sr.

where B,C,D,and E are the close curve fit coefficients determined by experimental data.The slip ratio sris defined as

Usually,in a real aircraft,the aircraft velocity can be measured by the following three methods.The first one is using an aircraft inertial navigation system(INS),which can provide the aircraft location,velocity,heading,and attitude.However,the INS data error increases as time changes,because they are generated by an integration method and the data precision of long-term use is poor.The second one is using an airspeed tube,which is an airspeed sensor and can be used to measure the velocity of the aircraft.However,the data error is very large especially when the aircraft in low-velocity cases.Moreover,the velocity measured by an airspeed tube is not the aircraft velocity relative to the ground,but the velocity relative to the atmosphere.However,the velocity needed in the braking process is the real aircraft velocity relative to the ground that an airspeed tube cannot accurately provide.It is difficult to

achieve accurate measurement by an airspeed tube.The third one is obtained from the front wheel speed sensor,which is the simplest and most effective way to get the aircraft velocity.The front wheel of the aircraft is equipped with a wheel speed sensor,which does not come with brake during aircraft taxiing.Therefore,the front wheel rolling linear velocity is the real aircraft velocity relative to the ground.In addition,in our experiments,the linear velocity of the inertia wheel actually is the aircraft ground velocity,and we can get the data from the speed sensor of the inertia wheel.The friction coefficient between the tire and the runway μris defined as

The vertical force Fvis given by

where FLis the lift force,Cyis the lift coefficient,S is the wing area,and ρa(bǔ)is the air density.Under a constant vertical force,the functional dependence between μrand srfor different runway conditions23is given in Fig.10.We can see that the maximum friction is achieved when the slip ratio is about 0.15 under the dry pavement condition.

The brake torque Tbis derived from the brake pressure Pbas

where fcis the friction coefficient,Rbis the equivalent radius,and Abis the piston area of the brake actuators.

4.2.ISBS model

The ISBS is a typical hydraulic system,which includes a constant pressure variable pump,an accumulator,a pressure control servo-valve,a brake actuator,and brake disks.The model of the oil tank is not considered in this paper,and it is assumed that the oil tank can supply the pump with sufficient oil.The actual actuator is equivalent to a hydraulic single-rod cylinder with a spring.The con figuration of the ISBS system is shown in Fig.11.

A pressure compensation valve is used to regulate the pressure of the swash-plate cylinder so as to control the displacement of the pump.The force equilibrium equation of the spool is simpli fied and represented as

where Psis the output pressure of the pump,Asis the area of the spool,Kvis the spring stiffness,and xvis the spool displacement.The dynamics of the spool is neglected because of the high spring stiffness and small mass of the spool.The flow of the pressure compensation valve Qvis related to the spool displacement xvand the pressure Ptas

where Kqm= ?Qv/?xvand Kcm=-?Qv?Ptare the flow gain and the pressure gain,respectively.Neglecting the external leakage,the flow continuity equation of the swash-plate cylinder is

where Atis the piston area,Vtis the volume of the piston chamber,Eyis the effective bulk modulus,and Ctis the total leakage coefficient.Similarly,neglecting the dynamics of the piston,the force equilibrium equation of the cylinder piston is

where Ktis the spring stiffness and xtis the piston displacement.The pump output flow Qsis related to the piston displacement xtand the pump rotation speed n as

where dzis the plunger diameter,Dzis the standard pitch diameter,Z is the number of plungers,and γ is the angle of the swash-plate.The accumulator model is derived from the state equation of ideal gas,which is

where P and V are the pressure and volume of the accumulator,P0and V0are the initial values of the accumulator parameters,k is the gas constant,and Qais the flow of the accumulator.In this paper,because of the high bandwidth of the pressure control servo-valve,the dynamics of the spool is simplified as a first-order model,which is

where Kiis the current gain coefficient,I is the control signal,Pbis the brake pressure,Alis the area of the servo-valve spool,Blis the combined coefficient of the damping and viscous friction forces,Klis the spring stiffness,and xiis the spool displacement.The output flow Qbis related to the spool displacement xiand the brake pressure Pbas

where Kqn= ?Qb?xiand Kcn=-?Qb?Pbare the flow gain and the pressure gain,respectively.Neglecting the external leakage,the flow continuity equation of the brake actuator is

where Abis the piston area of the brake actuator,Vbis the volume of the piston chamber,Cbis the total leakage coefficient,and xbis the piston displacement.The brake actuator is equivalent to a hydraulic single-rod cylinder with a spring,and its dynamics is simplified as

where Kbis the axial stiffness of the brake disks.The block diagram of the aircraft model and the ISBS model is shown in Fig.12.It can be seen that the aircraft model supplies the pump rotation speed n to the ISBS model,and gets the required flow Qffrom the ISBS model.

5.Feedback linearization controller design and simulations

In this section,a feedback linearization control algorithm is designed to ful fill the anti-skid control.In order to verify the validity of the controller and the ISBS,simulations are conducted in the MATLAB/Simulink environment.

5.1.Feedback linearization controller design

In order to simplify the design of the control method,Fd,Tp,and Mrin Eq.(5)are neglected,and the model of the pressure servo-valve is simplified as a proportional element,which is

where Kpis the gain coefficient of the brake pressure.We de fine the state variables x1=v and x2=ω,and de fine y as the control output of the system,so the control system is rewritten in a state space form as

where u is the control input,and we consider the control system in the following compact form:

Calculating the Lie derivative,we have

In conventional state feedback linearization,the objective is to find a state feedback and a diffeomorphism state transformation to transform a nonlinear system to an equivalent linear system in a canonical controller form.In the new coordinates,the nonlinearities of the control system can be removed by the feedback.We de fine a state transformation Z given by

and the derivative of the Z is

A control input is then obtained through

where udis the undetermined equivalent input under the linear system situation.Finally,the closed-loop system has a new set of variables in a linear state as

We de fine the desired output and get its derivative as

where v0is the initial velocity of the aircraft,and aris the desired deceleration of the aircraft.

Then we de fine the tracking error e=[e1,e2]as

and we have the derivative of the tracking error as

Thus,we can rewrite the control system as

where A and B are in a canonical form,which show that the new linear system is controllable;therefore,poles can be placed in the open left-half plane.In this paper,the nonlinearities are completely canceled,and the optimal linear quadratic regulator(LQR)with a full-state feedback is used for the linear system.The state feedback law ud=-Ke minimizes the quadratic cost function as

where Q and R are the weighting matrices which are positive and de finite,and set to diagonal matrices.The optimal control problem is formulated as the matrix algebraic Riccati equation,which is

Its solution P is needed to compute the optimal feedback gain K,which is

where k1and k2are the control parameters.Finally,we get the control law,which is

The system block diagram is represented in Fig.13.The control structure,according to Fig.13,has two feedbacks.The first feedback is responsible for system linearization,eliminating the existing linearity.The second feedback is the controller project based on the state feedback.

5.2.Simulations

A simulation is conducted to verify the feasibility of the ISBS.The aircraft is assumed to be taxiing at a landing velocity of 288 km/h(80 m/s).The ISBS starts to operate once the velocity of the aircraft is below 75 m/s,until the aircraft stops.The optimum slip ratio is about 0.15,which is almost equal to the maximum braking capacity.The time interval of data exchange is 5 ms which is consistent with the sampling time in the experiment.The results of the ISBS simulation are presented in Fig.14.

Fig.14(a)shows that the aircraft stops in about 32.5 s from an initial velocity of 288 km/h(80 m/s).The wheel linear velocity keeps an appropriate difference from the aircraft velocity without locking.Fig.14(b)is the slip ratio,and we can see that it always stays around 0.15 during the entire braking process.Fig.14(c)shows that the accumulator pressure rises sharply up to 21 MPa with the landing of the aircraft.The pressure drops from 21 MPa to 10 MPa when the aircraft speed is below 18 km/h(5 m/s).This is because when the aircraft speed is less than 18 km/h(5 m/s),the pump output flow will not be sufficient for the brake system,and then the accumulator supplies a flow to the brake system.Therefore,the accumulator pressure drops,but the pressure will not drop below the rated brake pressure,which is 6 MPa in this case,due to the matching design of the accumulator parameter in Section 3.

As illustrated in Fig.14(d),the brake pressure is quickly stabilized around 6 MPa as the aircraft starts to brake,and once the aircraft stops and the servo-valve is fully open,the brake pressure becomes consistent with the pressure of the accumulator.Before the aircraft starts braking,there is only the friction torque due to the friction between the tire and the runway surface,as shown in Fig.14(e).The brake torque rises in the same manner as the brake pressure when the aircraft starts to brake.Fig.14(f)indicates that the maximum value of the pump torque is about 3 N·m,which is much smaller than the brake torque,hence we can neglect the impact of the pump torque on the ISBS.The overall aircraft brake distance is 1312 m,as shown in Fig.14(g).The simulation indicates that the new system proposed in this paper can ensure sufficient oil consumption and feasible pressure for the brake system.

6.Experiments

A research prototype is designed and built according to the component parameters determined in Section 3,as shown in Fig.15.Its length is 200 mm,and width is 150 mm.The total mass is about 6.5 kg.All components are integrated in a hydraulic circuit block,including a 0.15 L oil tank.Two pressure sensors are used to measure the pressures at the pump output and the valve output.The cracking pressure of the security valve is 21 MPa.The ISBS requires about 3 N·m torque for the pump,which is much smaller than the brake torque.Therefore,the pump of the ISBS has little impact on the braking process,which also means that the ISBS only recovers a small part of kinetic energy of the landing aircraft.

The ISBS pump is connected to the main wheel by transmission gears,as shown in Fig.16,which explains how the ISBS is assembled together with the braking wheel in detail.Fig.16(a)is the virtual prototype of the ISBS,and the ISBS prototype is assembled together with the braking wheel by two screw holes,as shown in Fig.16(b),so that the transmission gear on the ISBS can mesh with the transmission gear on the braking wheel as shown in Fig.16(c).Then,after the aircraft landing,the rotating wheel can drive the hydraulic pump,generating high-pressure oil for the braking system.Fig.16(d)is the ISBS integrated schematic diagram with the braking wheel,and we can see that the ISBS can be easily assembled with the braking wheel.Since the ISBS only recycles very little power just enough for the braking system,its components specification and volume are very small compared to the braking wheel itself,and the installation space is sufficient.

To further test this novel brake system,an experiment is conducted on a ground inertia brake test bench.The structure of the ground inertia brake test bench and the assembly connection with the ISBS are shown in Fig.17.Its working principle is as follows.Firstly,a motor drives an inertia wheel rotating at the aircraft landing speed.The high-speed rotating inertia wheel is used to simulate the inertia of the landing aircraft.Then,the load device causes the aircraft wheel to engage the inertia wheel.As a result,the aircraft wheel rotates together with the inertia wheel.The force provided by the load device simulates the aircraft load.Finally,the hydraulic brake actuators generate the brake torque.The friction produced between the wheel tire and the inertia wheel stops the inertia wheel,so as to complete the brake function simulation.

The experimental results of the ISBS are shown in Fig.18.The aircraft lands at 9 s with a speed of 288 km/h,as shown in Fig.18(a),and the wheel linear velocity sharply increases from 0 km/h to 288 km/h in about 1 s.At the same time,the aircraft wheel load also sharply increases from 0 kN to 50 kN with the aircraft landing as shown in Fig.18(f),and almost remains constant in the whole braking process.More importantly,from Fig.18(c),we can also see that the accumulator pressure increases rapidly from 0 MPa to 21 MPa in about 1 s with the aircraft landing.It indicates that the build-up time of the accumulator pressure is very short,and it is entirely feasible to use the accumulator to maintain the brake pressure when the pump output flow is not insufficient.The pre-set pressure value of the constant pressure variable pump output is 21 MPa,so the maximum pressure value of the pump output and the accumulator is 21 MPa.

The aircraft begins to brake at 12 s with a speed of 288 km/h,as shown in Fig.18(a),and the wheel linear velocity keeps an appropriate difference from the aircraft velocity without locking up in the whole braking process.As can be seen from Fig.18(b),the slip ratio sharply rises to 0.15 at the start of the brake and is stabilized around 0.15,which is the optimum value of the slip ratio.Fig.18(d)indicates that the brake pressure also sharply rises to 8 MPa and keeps constant until the aircraft stops.As a consequence,the brake torque is stabilized around 9 kN·m so as to meet the requirements of the aircraft brake,as shown in Fig.18(e).However,it should be noted that the accumulator pressure is significantly lower at the start of the braking process,as can be seen from Fig.18(c).This is because the dynamic response of the accumulator is higher than that of the pump swash plate,namely,the accumulator first delivers flow to the brake system at the very start of the braking process,and then the pump swash plate starts responding to the system change.Obviously,the ISBS can improve the dynamic response performance of the hydraulic brake system by using an accumulator.Fig.18(g)shows that the temperature of the brake disks begins to go up when the aircraft starts to brake.

In the first half stage of the braking process,there is no problem with the ISBS,because the pump output flow is sufficient for the brake system at this stage.However,with the wheel angular velocity decreasing gradually,the pump output flow also drops down,and when the aircraft slows to 20 km/h,the accumulator,as the source of the hydraulic pressure,starts to supply flow to the brake system,because the pump output flow is not sufficient under a speed of 20 km/h,which can be seen from Fig.18(a)and(c).In addition,Fig.18(c)indicates that the aircraft velocity slows down from 20 km/h to full stop with the accumulator pressure decreasing from 21 MPa to 10 MPa,namely,the designed parameter of the accumulator is particularly appropriate.More importantly,in the second half stage of the braking process,both the brake pressure and the brake torque can keep the same values as those of the first half stage.The total brake time is about 28 s,and the total brake distance is about 1120 m.The experimental results indicate that the proposed ISBS is feasible and provides a new potential solution in the field of MEA brake systems.

7.Conclusions

This study presents a novel self-powered brake system for aircraft.It recovers a small part of kinetic energy of an aircraft during braking,and generates local hydraulic energy to brake.As a result,it makes hydraulic pipelines redundant while retaining mature hydraulic actuators.The entire system is compact and fully integrated,with all electrical interfaces,and can be situated in a landing gear next to a main wheel.The new system increases the security,economy,and reliability of an aircraft compared with conventional hydraulic braking systems,while having lower heat accumulation and better performance than those of electric brake systems.In addition,due to its self-powered characteristic,the ISBS has a failure-proof function under certain emergency circumstances such as EDP power interruption.The ISBS provides a new solution in the field of MEA brake systems.

Acknowledgements

The authors would like to acknowledge the supports from the Science and Technology on Aircraft Control Laboratory and Aviation Key Laboratory of Science.This study was cosupported by the National Natural Science Foundation of China(No.51475020)and the National Key Basic Research Program of China(No.2014CB046401).

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